1. Field of the Invention
The present invention generally relates to apparatus and methods used to transport hydrocarbons from a wellbore to another location. More particularly, the invention relates to a multiphase pump for removing hydrocarbons and other material from the wellbore.
2. Description of the Related Art
In a conventional onshore, under-balanced drilling operation, a wellbore is formed in the earth to access hydrocarbon bearing formations. During the drilling operation, a relatively light weight medium with a gas constituent is circulated through the wellbore to cool the drill bit and remove cuttings from the wellbore. The drilling material, gas, and cuttings, which are referred to here as “wellbore fluid” is circulated back to the surface of the wellbore. The wellbore fluid is then transported by a flowline to a separator where it may be separated into gas, liquids, and solids. If the wellbore fluid does not have adequate energy to flow to the separator, it may be pumped by a multiphase pump. These pumps are capable of moving volumes of the oil, gas, water, solids, and other substances making up the wellbore fluid. The multiphase pumps can be connected to a single or multiple wellheads through the use of a manifold. An exemplary multiphase pump is described in U.S. patent application Ser. No. 10/036,737, filed on Dec. 21, 2001, which is herein incorporated by reference in its entirety.
Currently, the under-balanced drilling operation requires at least one large separator to be present on location to handle the wellbore fluid during the drilling operation. The gas phase is separated and then usually flared or re-injected into the wellbore while the solid and liquid phases are captured for re-use and/or disposal. While the separator does its job effectively, it is costly to rent, transport, and personnel costs on location are high. Additionally, the physical size of the separator occupies valuable well site real estate that could be used for other necessary oilfield equipment.
There is a need therefore for more space and a cost efficient method and apparatus to handle gas bearing wellbore fluid.
In a conventional offshore drilling operation, a floating vessel and a riser pipe are used to connect surface drilling equipment to a sub-sea wellhead located at the sea floor. The riser pipe is typically filled with returning drilling fluid resulting in a relatively large hydrostatic pressure due to the length of the riser. This hydrostatic pressure in the riser, combined with additional pressure brought about by the circulation friction of the fluid, combines to form an equivalent circulating density “ECD”. In some instances, the ECD can exceed the fracture pressure of the formation adjacent the wellbore permitting drilling fluids to enter the formation. Permanent damage to the formation and loss of expensive drilling fluid is a typical result of fracturing the formation due to the effects of ECD.
The oilfield industry has attempted to solve the ECD problem in offshore drilling operations with an operation known as “pump and dump”. In this arrangement, the cuttings and mud used to drill the sub-sea wellbore are not returned in a riser but are separated at the sea floor. The mud is returned to the surface of the well via a separate line while the solids are allowed to flow out on to the seabed and remain there.
Recently, another method has been developed to reduce the effects of hydrostatic pressure in an offshore drilling operation. In one such arrangement, described in U.S. Pat. No. 6,505,691, filed by Judge on Aug. 6, 2001, a diaphragm type pump is used on the floor of the sea to transport drilling fluid, including solids to the surface of the sea. While the pump is capable of pumping solids and liquids, its volume is limited by its design requiring a high number of pump cycles to move a typical volume of fluid produced from the wellbore.
There is a need, therefore, for a cost effective method and apparatus to reduce the hydrostatic and ECD related pressures in an offshore drilling operation. There is a further need for a method and an apparatus to effectively return multiphase material to the surface while drilling a sub-sea well. There is yet a further need for a cost effective method and an apparatus for separating a gas portion of wellbore fluid from a liquid portion thereof.